This invention relates to a plating bath for depositing one or more metals selected from the group consisting of tin, lead, bismuth,, indium, gallium and germanium onto a metal surface such as on copper or copper alloys used for electrodes or various electronic circuit elements. More particularly, this invention relates to immersion plating of one or more of the above metals on copper, copper alloys, and other metals by chemical displacement using an immersion, a spray, a flood or a cascade application process. Still more particularly, this invention relates to the use of such plating solutions in the manufacture of printed circuitboards.
Coatings of tin, lead, bismuth, and alloys thereof have been applied to surfaces of copper and copper-based alloys by displacement plating such as by immersion plating techniques. Chemical displacement plating has been used in the manufacture of printed circuitboards (PCB's) and particularly multilayer printed circuitboards. Printed circuitboards generally comprise a non-conducting or dielectric layer such as a fiberglass/epoxy sheet which is clad with a metal conductive layer such as copper on one or both surfaces. The metal layer on the PCB, before processing, typically is a continuous layer of copper which may be interrupted by a pattern of plated through holes linking both surfaces of the board. During processing, selected portions of the copper layer are removed to form a raised copper circuit image pattern. Multilayer PCB's are typically constructed by inter-leaving imaged conductive layers such as one containing copper with dielectric adhesive layers such as a partially cured B-stage resin, i.e., a prepreg, into a multilayer sandwich which is then bonded together by applying heat and pressure. Production of these types of PCB's is described in "Printed Circuits Handbook," Third Edition, Edited by C. F. Coombs, Jr., McGraw-Hill, 1988, which is incorporated herein by reference. Since the conductive layer with a smooth copper surface does not bond well to the prepreg, copper surface treatments have been developed to increase the bond strength between the layers of the multilayer PCB sandwich. One example of a copper surface treatment is the use of immersion tin and tin alloys as a bonding medium for multilayer circuits as disclosed by Holtzman et al (U.S. Pat. No. 4,715,894). In this patent, an immersion tin composition is disclosed containing thiourea compounds and urea compounds to displacement plate the copper surface of each PCB with tin by the immersion process prior to laminating them to form a multilayer board.
U.S. Pat. No. 5,196,053 describes displacement tin plating, and more particularly, the use of a complexing agent which is an imidazole-2-thione compound. This complexing agent is utilized in place of thiourea which is described in the prior art. More particularly, U.S. Pat. No. 5,196,053 describes an aqueous plating solution for displacement plating of a substrate metal surface with another metal, and the plating solution comprises
(i) a metal ion of a free metal, wherein the free metal is different from the metal of the substrate surface; PA1 (ii) a complexing agent which is an imidazole-2-thione compound; and PA1 (iii) an acid. PA1 (A) at least one solution-soluble metal salt selected from the group consisting of a stannous salt, a lead salt, a bismuth salt, an indium salt, a gallium salt and a germanium salt; PA1 (B) at least one acid selected from the group consisting of fluoboric acid, alkane sulfonic acids, alkanol sulfonic acids, and mixtures thereof; PA1 (C) a complexing agent which is an imidazole-2-thione compound of the Formula III ##STR2## wherein A and B are the same or different --RY groups wherein R is a linear, branched or cyclic hydrocarbylene group containing up to 12 carbon atoms, and Y is a hydrogen, halogen, cyano, vinyl, phenyl, or ether moiety; and PA1 (D) water.
The acids that may be used include organic acids and inorganic acids based on sulfur, phosphorus, halogens, or mixtures thereof. The sulfur-based mineral acids are preferred, and examples of these include sulfuric acid and sulfamic acid. A mixture of sulfuric and hypophosphorous acids is particularly preferred. Examples of organic acids that can be used include monocarboxylic or dicarboxylic acids having up to about 6 carbon atoms such as formic acid, acetic acid, malic acid, maleic acid, etc.
U.S. Pat. No. 5,160,422 relates to plating baths for immersion tin-lead plating on copper or copper alloys. In addition to tin and lead, these baths contain an organic sulfo compound such as an alkane sulfonic acid or a hydroxy alkane sulfonic acid, and thiourea. Fluoborates also may be included in these plating solutions.